Forget chasing the No. 1 ranking. Phil Mickelson was more worried about being able to get out of bed.

Mickelson revealed Tuesday he has psoriatic arthritis, which causes his immune system to attack his joints and tendons and produced such intense pain he couldn't walk. He noticed the first symptoms five days before the U.S. Open, and the pain eventually got so bad he made a visit to the Mayo Clinic.

Weekly shots have brought the disease under control, and Mickelson said there shouldn't be any negative impact on his long- or even short-term health.

"I'm surprised at how quickly it's gone away, and how quickly it's been able to be managed and controlled," he said. "I feel 100 percent, like I say. But when I'm laying there on the couch and I can't move, you know, yeah, I had some concerns. But I feel a lot better now."

As if that bombshell wasn't enough for the day, Lefty dropped another one: the burger connoisseur is now a vegetarian.

"I know, I know," he said as reporters laughed. "As long as I believe that there's a possibility that it will help me overall, yeah, I'll continue to do that. If it will somehow keep this in remission or stop it from coming back, yeah, I'll be able to do it. But I haven't been put it to the real test. The real test is driving by a Five Guys and not stopping."

Mickelson has been threatening to replace Tiger Woods as world No. 1 since The Players Championship in early May, only to stumble at each opportunity. That's led many to wonder what's ailing him, particularly after his lackluster round Sunday at Firestone, where he made seven bogeys and a double-bogey - and just one birdie - on his way to a 78.

Turns out, the ranking was the least of his worries.

Mickelson, who turned 40 in June, said he woke up five days before the U.S. Open with "intense pain" in his tendons and joints that left him unable to move and his joints feeling sprained. Stretching, walking and anti-inflammatories alleviated the pain, and he went ahead and played Pebble Beach, where he shot 66 on Friday to put himself in contention. He wound up tied for fourth.

But the condition got progressively worse during the U.S. Open and a family vacation to Hawaii afterward, spreading to his knees, hips and elbows.

"That's when I got concerned," he said. "I certainly had the gamut of thoughts. ... I would just lay down and I couldn't roll over. I was concerned about being able to swing a club and so forth."

After the British Open, Mickelson made a trip to the Mayo Clinic in Rochester, Minn., where doctors confirmed the diagnosis of psoriatic arthritis. According to the Mayo Clinic website, the condition causes joint pain, stiffness and swelling. While it can be linked to psoriasis, the arthritis can appear without the presence of skin lesions.

There is no cure for the condition, according to the Mayo Clinic website, but the disease is treatable. Weekly shots of Enbrel lower his immune system, and Mickelson said the difference was noticeable almost immediately.

"I feel great now and things have been much, much better," he said. "I'll probably take this drug for about a year, and feel 100 percent. I'll stop it and see if it goes into remission and it may never come back. It may be gone forever."

"It's not that it's cured, but it may never come back," he added. "Or if it does come back, I'll start the treatment again and should be able to live a normal life without having any adverse effects. So I'm not very concerned about it."

The arthritis is the latest health scare for Mickelson's family. His wife, Amy, and mother are both battling breast cancer; the long-term prognosis for both is good.

While Mickelson said the arthritis didn't affect his play at Pebble Beach or St. Andrews, where he was never a factor, he only resumed his regular practice routine last week. Revealing his condition earlier might have lessened some of the criticism of his game these last few weeks.

But that's not Lefty's style.

"First of all, I don't want excuses. And second, I don't want to discuss something when I don't know what the outcome is going to be," Mickelson said. "For five or six weeks, I was a little unsure of how this was going to affect me long term, career, what have you. Now that I feel confident it's not going to affect not only the rest of my career or the rest of my life, but even in the short term it shouldn't have an effect, I feel a lot better about it and I'm a lot more at ease to discuss it."

Mickelson certainly looked at ease Tuesday during his practice round at Whistling Straits. He joked and laughed often with playing partners Dustin Johnson, Jeff Overton and Steve Marino, and it was clear there was more riding on the round than simple practice.

When Mickelson drained a 10-footer on 18, he gave a hearty fist pump.

"I'm probably not as sharp as I would like to be," the four-time major champion said. "I didn't play well at the British, obviously. I didn't play well last week, on the weekend, but I'm able to work on it. I had a good session with Butch and I believe that the game's coming around. I'm not sure where I'll be on Thursday, but hopefully I'll be ready."